NO302167B1 - Lightweight cement for use in high pressure environments - Google Patents

Description

This invention relates to the field of well cementing and, more particularly, a relatively light weight cement (less than 1300 kg/m<3>) for use in high pressure environments.

Within the field of well cementing, there is no lightweight cement available, i.e. a cement mixture with a density of less than approx. 1300 kg/m<3>, for use in environments with high pressure. Normal lightweight cement components, such as microsilica (microsilicon dioxide) and fly ash, do not provide cement densities that low while maintaining an acceptable compressive strength of greater than 3.45 MPa at temperatures below 50°C. Cements can only be formulated to achieve this low density using a combination of microsilica and ceramic microspheres. The difficulty in using such microspheres to obtain a lightweight cement mixture is that at pressures greater than 20.7 MPa the microspheres are crushed, any gas occurring inside the microspheres is compressed accordingly by the high pressure, and the density of the cement mixture increases.

The present invention provides a cement mixture which achieves low density using ceramic microspheres, but which, due to the synergistic effect of other cement components, avoids the crushing of the microspheres at pressures higher than 20.7 MPa, so that the lightweight nature of the mixture is maintained.

According to the invention, a lightweight cement comprises an aqueous hydratable cement mixture, ceramic microspheres and a foaming surfactant.

According to the invention, the above-mentioned cement mixture further comprises micro-silica.

It is therefore an object of this invention to provide lightweight cement which can be used at high pressure (greater than 20.7 MPa), but which maintains a density of less than approx. 1300 kg/m<3>. Such a lightweight cement mixture is specified in claim 1.

These and other objects according to the invention are achieved by the essence and form of the present invention, which shall be further described in connection with a description of preferred embodiments of the invention and in connection with the attached drawing which forms part of this description, where the only figure graphically illustrates the effect of the incorporation of microsilica on the compressive strength of the resulting cement formulated according to the invention.

The present invention will now be described in a more limited aspect of a preferred embodiment of the invention, including examples that illustrate the advantages of the present invention. It should be understood that the invention should not be limited only to the described embodiments, but have considerably wider applicability in the technique.

It has surprisingly been found that the effects of high pressure in crushing ceramic microspheres in a cement mixture with the resulting increase in cement density can be overcome by the incorporation of relatively small amounts of a foaming surfactant into the cement mixture.

As used in this specification and in the claims, the term "foaming surfactant" shall be considered to mean any surfactant which, if combined in aqueous solution with a gas, results in a relatively stable dual-phase system, where the continuous phase is water (including the surfactant) and the discontinuous phase is gas. Such surfactants include, but are not limited to, "anionic, cationic, or nonionic compounds, such as sulfonated alkoxylates, alkyl quaternary amines, ethoxylated linear alcohols, and the like. One particularly useful surfactant that has been found to be highly effective in the mixtures according to the present invention, comprises an ammonium fatty alcohol ether sulfate with 12-30 carbon atoms in the fatty alcohol chain. According to the invention, the foaming surfactant is added to lightweight cement mixtures in a range of about 1 1 - about 20 1 of foaming surfactant with 85% active content per metric ton of dry cement mixture (approx. 0.05 - approx. 1% by weight).

The lightweight cement mixtures where the surfactant according to the present invention exhibits a synergistic effect comprise a hydratable cement, such as cements of API class G, class H, class A or class C, ceramic microspheres with a particle size in the range of approx. 40 - approx. 200/im, and which has a specific gravity in the range of approx.

0.5 - approx. 0.9. Optionally, a preferred cement mixture further comprises microsilica with a particle size in the range of approx. 0.01 - .approx. 1/xm in the dry mixture in amounts that are in the range of approx. 2 - approx. 200 or more kilograms of microsilica per tons of dry, mixed cement. The cement mixture may also optionally comprise a rheology-regulating agent, such as cellulose polymer or a sodium salt of condensed polynaphthalene sulphonate (sodium PNS) in a range of 0 - approx. 1% by weight of cement.

Example 1

A lightweight cement with a nominal density of 1200 kg/m<3> was prepared by mixing 500 kg of Class G cement, 125 kg of microsilica, 375 kg of ceramic microspheres, 0.75% by weight, based on cement, of sodium PNS and 9 1 per tonne dry mixture of an ammonium fatty alcohol ether sulphate with an active content of 82.5% as a foaming surfactant. This mixture was mixed with 0.8 ml of water. The results of tests of this mixture are shown in the table. Scanning electron micrograph examination of samples exposed to 41.4 MPa for 48 hours at 70°C shows that an estimated fraction of less than 1% of the ceramic microspheres were crushed, most spheres were intact, and less than 10% showed signs of on cracks or crushing. Shattered and broken spheres appeared to have occurred during the preparation of the sample.

Example II

A nominal 1100 kg/m<3>cement mixture was formulated, comprising 400 kg of Class G cement, 100 kg of microsilica, 500 kg of ceramic microspheres and 9 L of ammonium fatty alcohol ether sulfate with an active content of 82.5% as a foaming surfactant per ton of dry mixture together with 1 m<3> of water/tonne. The results of tests of this mixture are shown in the following table. Scanning electron micrograph examination of samples exposed to 41.4 MPa for 48 hours at 70°C shows that an estimated fraction of less than 1% of the ceramic microspheres were crushed, most spheres were intact, and less than 10% showed signs of on cracks or crushing. Shattered and broken spheres appeared to have occurred during the preparation of the sample.

Fig. 1 shows the effects of embedding microsilica on the compressive strength of the resulting cement mixture in a mixture corresponding to Example I. A skilled person can thus select the desired compressive strength for the resulting lightweight cement by adding microsilica to the mixture according to Example I according to the additive quantities shown in the graphic presentation in fig. 1.

Claims (3)

1. Lightweight cement mixture for high pressure environments exceeding 20.7 MPa, comprising an aqueous slurry of a hydratable cement and ceramic microspheres, and optionally microsilica and optionally a rheology-regulating agent, characterized in that the cement is selected from the group consisting of cements of class G, H, A and C, that the ceramic microspheres have a particle size in the range 4 0 - 200/xm and a density in the range 0.5 - 0.9 g/l, that the possible microsilica has a particle size in the range 0.1 - 1/xm, and that the possible rheology-regulating agent is selected from the group consisting of cellulose polymers and the sodium salt of polynaphthalene sulphonate, and that the cement mixture also comprises a foaming surfactant which is present in an amount in the range of 0.05 - 1% by weight of the dry mixed components. 2. Cement mixture according to claim 1, characterized in that the surfactant comprises an ammonium fatty alcohol ether sulfate. 3. Cement mixture according to claim 1, characterized in that the surface-active agent is selected from the group consisting of alkoxy quaternary amines, ethoxylated linear alcohols and sulphonated alkoxylates.